Silica, otherwise known as silicon dioxide, is a wellknown material with desirable properties. Silica has many known uses, such as but not limited to, glasses, ceramics, abrasives, semiconductors and semiconductor devices.
Several different processes can be used to produce silica. The particular process used to produce silica depends on the intended end use and desired purity of the end silica composition.
Some silica compositions possess substantial optical clarity, almost to the point of being a transparent silica composition. A transparent silica material can be made opaque if, for example the silica material contains light scattering centers, such as voids.
However, if an end use of a porous opaque silica necessitates the machining, cutting or otherwise working of the porous opaque silica, voids therein may be exposed on the machined or cut surface. Further, the silica itself may be damaged by the machining, cutting and working process, especially at the voids, where there exists a substantial possibility of cracking, fracture or other undesirable side effects due to the machining, cutting and working process.
The machining, cutting or otherwise working the porous opaque silica results in open porosity on the machined or cut surface of the porous opaque silica, where the surface most probably comprises exposed and damaged voids on the machined or cut surface of the porous opaque silica. This exposed surface with exposed and damaged voids, otherwise known as surface defects, on the machined or cut surface of the porous opaque silica is not desirable, since the exposed, damaged and impaired voids will trap and hold foreign matter. Therefore, porous opaque silica, as it is now produced, is not well suited for some applications, especially where cutting, working or machining of the porous opaque silica is needed.
Performance of porous opaque silica, resulting from machining or otherwise working the porous opaque silica, may be initially adequate for some porous opaque silica applications. For example, porous opaque silica may be initially adequate for applications, such as but not limited to, heat baffles, infra-red blockers and flanges, which are commonly used in the semi-conductor processing industries. However, during use the parts made from porous opaque material can develop undesirable characteristics as the surface of the part erodes and exposes new void surfaces.
For example, in a heat baffle application of porous opaque silica, porous opaque silica may initially perform acceptably. However, in a heat baffle application of porous opaque silica, continued and repeated acid etching, which is for cleaning and removal of impurities that have been deposited as a result of an end-user's process, of the porous opaque silica will result in a substantial amount of open porosity on an exposed surface of the porous opaque silica. The open porosity on the exposed surface of the porous opaque silica can trap undesirable debris, dirt and other foreign matter. This trapping, of course, is undesirable since the foreign matter may significantly alter properties of the porous opaque silica. Accordingly, porous opaque silica is not an ideal and desirable material for a heat baffle application.
Further, a porous opaque silica is not an ideal material for a sealing application, for example in a flange usage. Machined silica parts are often subjected to a finishing step. The finishing step comprises at least one step selected from glazing and flame polishing. Glazing comprises impinging a hot flame to the part, thereby melting a surface layer of the part. The glazed surface of the finished part should be mostly free of porosity. However, there are often at least two problems associated with achieving a glazed surface free of porosity. First, repeated etching can open up a closed pore and cause the surface to have porosity. Second, if the pores in the material are large, even a glazed surface would be "bumpy." Such a "bumpy" surface would not provide a good vacuum sealing surface. Therefore, machined porous opaque silica or finished porous opaque silica is not normally ideal for sealing purposes in a flange application.